1. Field of the Invention
The present invention relates to a surface acoustic wave device for use as a resonator, a band filter, or other suitable device, and more particularly, the present invention relates to a surface acoustic wave device which has a configuration including at least two stages.
2. Description of the Related Art
According to the related art, there have been proposed various surface acoustic wave devices in which at least two surface acoustic wave filter elements are cascaded so that the attenuation is increased. FIG. 9 is a plan view of an example of such a surface acoustic wave device.
The surface acoustic wave device 101 contains a piezoelectric substrate 102. A longitudinally coupled type first surface acoustic wave filter element 103 and a longitudinally coupled type second surface acoustic wave filter element 104 are disposed on the piezoelectric substrate 102. The surface acoustic wave filter elements 103 and 104 are electrically connected to each other via an inter-stage connecting portion 105. The interdigital transducers and the reflectors which constitute the surface acoustic wave filter elements 103 and 104 and the inter-stage connecting portion 105 are made of a metallic material such as Al.
Referring to surface acoustic wave devices in which at least two surface acoustic wave elements are connected to form a configuration with at least two stages such as the surface acoustic wave device 101, problems have occurred in that a surface acoustic wave excited in an surface acoustic wave element is leaked and transmitted to another surface acoustic wave element, which deteriorates the characteristics of the device. For example, the attenuation in the frequency region which is out of the band (hereinafter, referred to as an out-of-band region), is decreased. Thus, in some cases, the desired effects of the configuration including at least two stages can not be obtained.
According to Japanese Examined Utility Model Registration Application Publication No. 63-12930 (Patent Literature 1), to solve the above-described problems, a surface acoustic wave device having a sound-absorbing material arranged between multi-mode surface acoustic wave filters is disclosed. Moreover, according to Japanese Unexamined Patent Application Publication No. 10-190404 (Patent Literature 2), a surface acoustic wave device provided with a resin pattern having a high absorptivity for a surface acoustic wave, the resin pattern being located between transversely-coupled type multi-mode surface acoustic wave filters having an at least two-stage configuration is disclosed. Furthermore, according to Japanese Unexamined Patent Application Publication No. 7-66678 (Patent Literature 3), a surface acoustic wave device in which a sound-absorbing material is coated between adjacent surface acoustic wave filter elements is disclosed.
Moreover, according to Japanese Unexamined Patent Application Publication No. 5-102783 (Patent Literature 4), a surface acoustic wave device having at least two surface acoustic wave elements which are cascaded is disclosed. In this device, a groove is formed on the surface of a piezoelectric substrate of the device, so that a surface acoustic wave can be reflected from and scattered over the groove.
According to the above-described configurations in which the sound-absorbing material, the resin pattern, or the like is provided between surface acoustic wave elements, an area for locating the sound-absorbing material and so forth is required. Thus, increases in the size of such a surface acoustic wave device can not be avoided. Moreover, additional materials such as the sound-absorbing material or the like are required, and also, a process of coating the sound-absorbing material or the like must be carried out. Thus, there have been problems in that the production efficiency becomes low, and the cost is increased.
Furthermore, according to the configuration in which a groove is formed between adjacent surface acoustic wave elements on a piezoelectric substrate as disclosed in Patent Literature 4, increasing the size of the device can not be avoided, since the groove is formed. In addition, the piezoelectric substrate may be cracked or cut during formation of the groove. Furthermore, a process of forming the groove needs to be carried out. Thus, there have been problems in that the production process for the device is complicated, and the non-defective ratio is reduced.